Stone walls are a feature of many of our towns and cities and provide a unique and demanding habitat for plants. Some plants do manage to grow in this situation in spite of adverse features such as lack of soil, intermittent water supply and human interference.
Studies of walls in England described a variety of species, but there is less information for Scotland. A small study of towns in Easter Ross described 64 species on walls (Ballinger 2014).
Method: 50 mortared stone walls of at least 2m height near my home in West Dundee were studied on 3 occasions in April, May and June 2020. Sections of approximately 100m were examined and all vascular plants noted.
Results: 54 species were recorded on the walls, the commonest being; Asplenium trichomanes (Maidenhair Spleenwort) (40 walls), Asplenium ruta-muraria (Wall-rue) (37 walls), Cymbalaria muralis (Ivy-leaved Toadflax) (25), Taraxacum agg. (Dandelions) (21), Epilobium species (Willowherbs) (20), Buddleja davidii (Buddleja) (15) and Linaria purpurea (Purple Toadflax) (14).
In those parts of the wall 0-1m above the ground 22 species were found, in the 1-2 m above the ground sections 38 species were seen and the tops of the walls yielded 31 species.
Of the commoner species, Asplenium trichomanes was recorded on 36 of the 1-2m sections and on 20 of the 0-1m sections. The corresponding figures for Asplenium ruta-muraria were 37 and 18.
Numbers were limited in relation to the aspect of the walls but there was a suggestion that small ferns did not do as well on south facing walls.
33 species were found in the 1cm. of ground immediately adjacent to the base of the wall (these are not included in the above wall totals). The commonest were Poa annua (Annual Meadow-grass), Epilobium (Willowherb) species and Arabidopsis thaliana (Thale Cress). 15 of the species growing at the base of the walls were not found at all on the stone walls above in this sample.
Comment: This is a limited survey in a local area and a wider investigation would have produced a longer species list. However, it does show that some vascular plants manage to grow on these mortared walls, as other studies have also demonstrated in the past. Some species such as the small ferns are wall specialists, whereas others put in occasional casual appearances.
The reason for the greater number of species further up the wall found here is not clear. It may relate to herbicide use on the base, salt treatment of roads and pavements, pollution from traffic and weeding by owners. Many of the roads studied were busy thoroughfares.
I propose that we should value our town wall vegetation and do our best to limit any damage to the plants that grow there.
Reference: Ballinger B. (2014): Some man-made habitats in Easter Ross, BSBI NEWS No 126: 11-12
List of plants on walls: Agrostis sp., Arabidopsis thaliana, Asplenium adiantum-nigrum, Asplenium ruta-muraria, Asplenium trichomanes, Asplenium (Phyllitis) scolopendrium, Antirrhinum majus, Aubretia deltoidea, Betula sp., Buddlleja davidii, Campanula poscharskyana, Cardamine hirsuta, Centranthus ruber, Cerastium fontanum, Cerastium glomeratum, Cerastium tomentosum, Chamaecyparis lawsoniana, Chamerion angustifolium, Cochlearia danica, Cotoneaster horizontalis agg., Cotoneaster spp., Cymbalaria muralis, Dactylis glomerata, Digitalis purpurea, Dryopteris spp., Epilobium ciliatum, Epilobium montanum, Erinus alpinus, Festuca rubra, Galium aparine, Hedera helix s.l., Helleborus foetidus, Hieraceum agg., Lamium pupureum, Linaria purpurea, Mahonia sp., Meconopsis cambrica, Mycelis muralis, Papaver rhoeas, Parietaria judaica, Poa annua, Poa pratensis, Polypodium vulgare, Pseudofumaria lutea, Sagina procumbens, Sambucus nigra, Sedum forsterianum, Senecio jacobaea, Senecio vulgaris, Sonchus oleraceus, Stellaria media, Tanacetum parthenium, Taraxacum agg., Viola riviniana.